Electronic devices such as notebook, computers and smartphones usually include a variety of memories to store data. The types of memories can be divided into two categories: volatile memories and non-volatile memories. Data stored in volatile memories will be lost after the power of the volatile memories is removed. On the contrary, data stored in the non-volatile memories may retain even if the power is removed. Volatile memories include random access memory (RAM), which may be further divided into two sub-categories: static random access memory (SRAM) and dynamic random access memory (DRAM).
An SRAM memory unit is constituted by a plurality of SRAM cells. Each of the SRAM cells may include different numbers of transistors. An SRAM cell consisting of six transistors is referred to as a six-transistor (6-T) SRAM, for example. In an SRAM memory chip, SRAM cells may be arranged in rows and columns. An SRAM cell is selected during either a read operation or a write operation by selecting the cell's row and column. In manufacturing of the SRAM, each of the SRAM cells is designed to work at a specific voltage. Only in that specific voltage can the SRAM work at an expected speed and accuracy.